1. FIELD OF THE INVENTION
The present invention relates to a gas component detection apparatus for detecting with high sensitivity and rapid response variations in concentrations of gas components such as oxygen (O.sub.2), carbon monoxide (CO) and hydrocarbon (HC) contained in a gaseous mixture. The invention finds its useful application particularly in a gas component detection apparatus for an exhaust gas of an internal combustion engine.
2. DESCRIPTION OF THE PRIOR ART
Gas component detection apparatuses have been widely used in many industrial fields. Lately, as a counter measure to cope with the problem of an exhaust gas of internal combustion engine, the gas component detection apparatuses are employed for determining an air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine.
Heretofore, an electric cell comprising a solid electrolyte such as zirconia has been employed for the detection of the air-fuel ratio of air-fuel mixture supplied to the internal combustion engine. More particularly, the variation in the electromotive force of the cell depending on the concentration of oxygen contained in the exhaust gas was utilized as a measure for indicating a corresponding variation in the air-fuel ratio. Further, it has been also known to employ a semiconductive metal oxide and detect the variation in the electric resistance thereof as a measure to represent the variation in the air-fuel ratio.
In the case of the solid electrolyte cell, the electromotive force is generated by the migration of ions through the lattice defects of the solid electrolyte under thermal excitation, so that when the temperature of the solid electrolyte is lower than 400.degree. C no electromotiveforce is generated. Accordingly, the gas component detection apparatus utilizing such solid electrolyte has a serious drawback such that the response of the apparatus is remarkably lowered when the temperature of the exhaust gas is low as is the case of starting the internal combustion engine.
On the other hand, the detection apparatus utilizing the variation of electric resistance of a semiconductive metal oxide suffers from a disadvantage such that the detection of variation in the air-fuel ratio with respect to a predetermined value can not be effected with a desired accuracy since the characteristic curve representing the resistance variation of the semiconductive metal oxide relative to the variation in the air-fuel ratio has a relatively gentle inclination. Furthermore, when the gas component detection apparatus incorporating the semiconductive metal oxide as the detecting element is employed for determining the air-fuel ratio of the air-fuel mixture supplied to the engine, there arises the following problems. Namely, the surface of the semiconductor detecting element exposed to the exhaust gas during usage of the detection apparatus is deposited and gradually accumulated with poisonous substances such as phosphorus (P), lead (Pb), sulfur (S) and compounds thereof contained in the exhaust gas in addition to H.sub.2, CO, HC, O.sub.2, whereby the material constituting the detecting element will react with such poisonous substances to form compounds, incurring deterioration of the performance of the semiconductor detecting element. In such case, the characteristic curve of the detecting element is adversely changed. Further, when a catalyst is carried on the surface of the metal oxide constituting the detecting element, the performance or activity of the catalyst is also subjected to deterioration.